Welcome! It's great to see that G.A.S. is an international phenomenon
On any gravely walker, if you have a situation where the attachment wedges up, but the rest of the drivetrain works, especially fwd/reverse, the issue has to be in the attachment clutch.
A stock C12 has the old-style four-bolt attachment mount, but the pic of the clamshell deck looks like it's set up for a quick-hitch. If it's the latter, I'm pretty sure the clutch is internal to the QH adapter. (All of my gravely gear is older, so not positive)
If you get a close-up of where/how the attachments bolt to the nose of the tractor, we'll be able to tell more.
If it's the old-style clutch, adjustment is pretty easy, just tighten the clutch bolts. If it's a newer style QH, I'll defer to other members here who know that system better.
Price is all over the place. Given the rarity of those machines in Germany, that may well be the right price for a running machine with attachments.
Is it a good machine? I think so
I bet you won't find anybody here disagreeing. The gravely transmission is very robust, and the cast iron Kohlers are pretty bulletproof.
Good luck, and keep those cards and letters coming!